The present invention relates to an ion source device, and more particularly to an ion source device suitable for generating reactive ions.
In the ion source device, particularly in a high power ion source device, various discharges such as glow discharge, arc discharge and RF discharge are made in a low pressure discharge chamber to ionize gas in the discharge chamber so that ions are taken out of the plasma.
In such an ion source device, magnetic cusp fields are formed in the discharge chamber in order to generate plasma having a highly spatial uniformity. (See JP-A-No. 56-79900 and JP-A-No. 57-185653.)
In such a prior art device, an arc discharge is made by using a filament arranged in a plasma vessel as a cathode and a wall of the plasma vessel as an anode to ionize introduced gas, and the plasma is confined in the space in the vessel by utilizing the magnetic cusp fields so that they are effectively utilized. Magnetic characteristics of permanent magnets for generating the magnetic cusp fields degrades. In order to prevent the degradation of the magnetic characteristics of the permanent magnets for generating the magnetic cusp fields, the plasma vessel is cooled to prevent the temperature of the permanent magnets from rising too high.
However, in the ion source device utilizing the magnetic cusp fields, when a compound gas (fluorine or chlorine compound) is ionized to generate the plasma, electrical insulative high molecular product deposits on a wall of the plasma vessel by plasma polymerization reaction of the compound gas. Since the plasma vessel is an anode which is a positive electrode for a DC arc discharge, the discharge becomes unstable or discharge may be stopped and a basic operation of the ion source is interrupted and a stable discharge to the compound gas cannot be maintained.